The basic operation and structure of land mobile radio communication systems and cellular radio/telephone communication systems (communication systems) are known. Communication systems typically comprise a plurality of communication units (vehicle mounted or portable radios in a land mobile system and radio/telephones in a cellular system), a predetermined number of repeaters, which are located throughout a geographic region, that transceive information via communication channels, and a controller. Of the communication channels which may be TDM slots, carrier frequencies, a pair of carrier frequencies or other radio frequency (RF) transmission medium, one typically acts as a control channel that transceives operational data between the plurality of communication units and the controller.
A fundamental difference between land mobile communication systems and cellular radio/telephone communication systems is the coverage area of each repeater. In a land mobile communication system, the coverage area of a single repeater may be up to a thirty (30) mile radius, while the coverage area of a repeater in a cellular radio/telephone communication system is typically less than a two (2) mile radius. The relatively large coverage area of a land mobile system allows the land mobile system to readily handle group, or dispatch calls. (A group or dispatch call is one in which one communication channel is allocated to a group of communication units, each of which can communicate to the other units via the allocated channel.) In contrast, the relatively small coverage area of the cellular radio/telephone communication system allows the system to efficiently process private, or point to point, calls. (A private call is one in which a communication channel is allocated to a small number of communication units, typically two, for a private communication.) Although each system processes a particular type of call more efficiently, either system can perform a private call or a dispatch call.
In a land mobile communication system, the plurality of communication units are arranged into communication groups, or talk groups. A communication unit in a particular talk group may initiate a dispatch call by pressing a push-to-talk (PTT) button which informs the controller that a communication channel is needed for this particular talk group. If a communication channel is available, the controller allocates it to the particular talk group and sends out a message on the control channel to the plurality of communication units. The communication units in the particular talk group, after receiving the message, affiliate themselves with the allocated communication channel such that each member of the particular talk group can participate in the dispatch call. The number of communication units in any one dispatch call could range from just a few communication units to a few hundred communication units.
As is also known, a communication system may serve a large population of communication units and comprise a plurality of repeater sites over a large geographic region. During a dispatch call, individual communication units of the particular talk group may be located anywhere in the multi-site coverage area of the system. To efficiently process the dispatch call, the system must be able to set-up the communication paths between the all the sites, or in the sites having a member of the particular talk group located within it, quickly.
A typical prior art multi-site communication system infrastructure comprises, within each repeater at each site, an individual circuit that couples the repeater to the controller or central radio system switching point (central switching point). At each site, at least one repeater is permanently connected to the central switching point. When a dispatch is initiated, the dedicated repeater in each site is automatically connected together such that all members of the talk group could partake in the dispatch call. By having a dedicated repeater at each site for dispatch calls, the set-up of communication paths between each site was relatively fast, however, the overall efficiency of the system is reduced due to the dedication of repeaters to only dispatch calls.
To eliminate the dedication of repeaters to dispatch calls, another typical prior art multi-site communication system incorporates a switching circuit that only links repeaters in sites that have at least one member of the dispatch call located within its coverage area. The linking of repeaters on a call by call basis improves the system's overall efficiency because all of a site's repeaters can be used for any type of call, including dispatch calls. However, set-up times are longer for this type of system as opposed to system having dedicated repeaters since circuit set-up is necessary for each call. Another drawback to this type of system, as well as to the dedicated repeater system, is that both systems require each repeater, via individual circuits, to be linked to the central switching point. Such architectures, which are commonly called single-star architectures, are costly and waste communication links in comparison with distributed tree-structures.
In a distributed tree-structure, paths are only required between repeater sites such that a path exists from any repeater to any other repeater. A distributed switching system eliminates the need for an individual circuit to connect each repeater with the central switching point, however, set-up times for dispatch calls are relatively long due to the multiple switching connections between sites that need to be made.
Distributed switching systems have proven to be relatively efficient at processing point-to-point communications. In such communications, a sending communication unit transmits, via the repeater site that it is located in, a data packet to the destination communication unit. The packet is transmitted from the repeater site of the sending communication unit through the distributed tree-structure to the repeater site of the receiving communication unit. For point-to-multipoint communications, the repeater site of the sending communication unit has to individually transmit the packet to each repeater site that has a receiving communication unit located within it or transmit the packet to all the repeaters simultaneously. In either case, the links that couple the repeater sites together are inefficiently used.
Regardless of the communication system infrastructure (single star or distributed tree), once a dispatch call is established, prioritization of communications is the same. Prioritization is typically based on the type of call (e.g. an emergency call receives a higher priority than a routine check-in call) or the source of the communication (e.g. a console operator has a higher priority than an operator of a communication unit). In many applications, this prioritization scheme is adequate, however, as the size of the communication systems increase, it becomes more apparent that communications within a dispatch call may have little or no interest to a particular operator of a communication unit due to the physical separation of the communicators.
From the above, it is clear that a need exists for a multi-site communication system that provides for fast set-up of point-to-multipoint dispatch call connections between repeaters without requiring a single-star configuration and provides for more individualized prioritization of communications.